An ESD protection device is generally integrated in a semiconductor device, to protect the semiconductor device provided with an integrated circuit from an ESD surge. The ESD protection device needs to use active elements such as a transistor and a diode, to attain an active operation like a variable resistance.
In a general semiconductor device, because an active element is formed in the semiconductor substrate (e.g. a silicon substrate), the transistor or the diode, which is formed on the semiconductor substrate, is used for the ESD protection device. FIG. 1 is a sectional view showing an example of the structure of such a semiconductor device. The semiconductor device 100 of FIG. 1 is composed of a logic region 100A and the ESD protection device region 100B.
The logic region 100A is a region where a logic circuit and an integrated circuit are formed. In detail, a semiconductor element 102 such as a MOS transistors is formed in a part of the logic region 100A of a semiconductor substrate 101, and a plurality of wiring layers (6 wiring layers in FIG. 1) 103 are provided above it. Each of the wiring layers 103 is composed of wiring lines 104 and an interlayer insulating film 105 which electrically separates the wiring lines 104 from wiring lines of the adjacent wiring layer 103. The semiconductor element 102, the wiring lines 104 of the lowermost wiring layer 103 are the wiring line 104, and the wiring lines 104 of the two neighbor wiring layers 103 are electrically connected by via-contacts 106 provided to pass through the interlayer insulating films 105. The integrated circuit is formed from the semiconductor element 102, the wiring lines 104 and the via-contacts 106.
On the other hand, the ESD protection device region 100B is a region where the ESD protection device 107 is formed. The ESD protection device 107 is formed in a part of the ESD protection device region 100B of the semiconductor substrate 101. An active element is used for the ESD protection device 107, and a thyristor having a PNPN structure is formed as the ESD protection device 107 in an example of FIG. 1. The ESD protection device 107 is connected with an I/O pad and a ground pad provided in the uppermost wiring layer 103 through the wiring lines 104 and the via-contacts 106 provided in each wiring layer 103. In FIG. 1, the wiring lines connected with the I/O pad are shown by a reference numeral 108 and the wiring lines connected with the ground pad are shown by a reference numeral 109. When an ESD surge is applied to the I/O pad, the ESD protection device 107 is turned on to pass the ESD surge to the ground pad. The internal integrated circuit is protected from the ESD surge through such an operation.
As shown in FIG. 1, one problem of the semiconductor element, which uses the transistor or diode formed on the semiconductor substrate, as the ESD protection device is to cause the increase of the chip area for forming the ESD protection device. Because the ESD protection device is formed on the semiconductor substrate, the area to form the ESD protection device must be provided separately from the area for forming the integrated circuit. This leads the increase of the chip area. Moreover, when the ESD surge is applied, the transistor or diode having a large area must be formed as the ESD protection device because a large amount of current can flow through the ESD protection device. This makes a problem of the increase of the chip area increasingly more serious.
It should be noted that JP 2010-141230A discloses a technique of forming a semiconductor device using a semiconductor layer provided in a wiring layer, as the technique related to the present invention. As a material of the semiconductor layer, oxide semiconductor such as InGaZnO (IGZO) and ZnO, polysilicon, and amorphous silicon are exemplified. As an application of the semiconductor device provided in the wiring layer, a transistor as a switching element is exemplified. Also, a technique is disclosed in which a trap film and a back gate electrode are provided for the semiconductor device and the semiconductor device is used as a memory device. JP 2010-141230A describes nothing about the ESD protection.
Moreover, JP 2010-41058A, JP 2010-98280A and JP 2010-135762A disclose thin film transistors having oxide semiconductor films. In the techniques disclosed in these references, the thin film transistor having an oxide semiconductor film is used for active matrix display unit such as a liquid crystal display.